1. Field of the Invention
The present invention generally relates to motor control and image forming and, more particularly, to a motor control apparatus that controls a direct current (DC) motor, and an image forming apparatus that includes the motor control apparatus.
2. Description of the Related Art
There are copying machines and printers which form a toner image on a photosensitive drum, transfer the toner image to an intermediate transfer belt, and then transfer the toner image from the intermediate transfer belt to a recording material. Such copying machines and printers may be configured so that the photosensitive drum and the intermediate transfer belt are driven by separate motors to achieve high image quality.
The photosensitive drum and the intermediate transfer belt in the above-described copying machines and printers are in contact with each other at a transfer position. The photosensitive drum and the intermediate transfer belt may thus become scratched if there is a difference in peripheral speeds thereof.
Such a problem does not occur when the peripheral speeds of the photosensitive drum and the intermediate transfer belt are controlled at target speeds. However, if the photosensitive drum and the intermediate transfer belt are driven by separate DC motors, start-up of driving each of the DC motors becomes different and thus unstable.
There are various methods of stabilizing the start-up of driving the DC motor. Japanese Patent Application Laid-Open No. 7-039181 discusses increasing, when starting to drive the motor, a pulse width modulation (PWM) signal that controls rotation speed of the motor. The PWM signal is increased from a low on-duty ratio at a predetermined rate to start driving the DC motor in a shorter time with less overshoot.
Further, Japanese Patent Application Laid-Open No. 2007-156238 discusses controlling a clock frequency input to the motor to be lower than a target frequency when starting to drive the DC motor. The clock frequency is then increased to the target frequency.
If control is performed when driving of the motor is started so that the control value is increased from a low control value to the target control value, an angular speed of the DC motor linearly increases to a certain point. However, the angular speed increases non-linearly when approaching the target angular speed. As a result, the peripheral speed difference is generated between the photosensitive drum and the intermediate transfer belt during the period of non-linear increase.